The development of high-brightness LEDs has led to use of such devices in various lighting fixtures. In general, an LED-based lamp operates in a fundamentally different way than an incandescent, or gas discharge lamp, and therefore may not be connectable to existing lighting fixtures designed for those types of lamps. A ballast circuit may be used, however, to allow use of an LED-based lamp as a retro-fit for existing lighting fixtures.
The ballast circuitry for an LED-based lamp generally converts an alternating current (AC) input, such as a 120V/60 Hz line input or input from a dimmer switch, to a stable direct current (DC) voltage used for driving the LED-based lamp. Such circuitry may incorporate a rectifier for receiving the AC input and a DC-DC converter circuit. The DC-DC converter circuit may receive an unregulated DC output from the rectifier and provide a stable, regulated DC output to the LED-based lamp.
A variety of DC-DC converter configurations are well-known in the art. Certain types of known DC-DC converter configurations, such as buck converters, boost converters, buck-boost converters, etc., are generally categorized as switching regulators. These devices include a switch, e.g. a transistor, which is selectively operated to allow energy to be stored in an energy storage device, e.g. an inductor, and then transferred to one or more filter capacitors. The filter capacitor(s) provide a relatively smooth DC output voltage to the load and provide essentially continuous energy to the load between energy storage cycles.
One issue with such switching regulator configurations is that there may be no protective isolation between the unregulated DC voltage and the regulated DC output voltage. In some configurations, the unregulated DC voltage may be 400 Volts or more. The unregulated DC voltage can be dangerous if inadvertently applied to the load.
To provide protective isolation, therefore, a transformer-based switching regulator, such as a known “flyback” converter, may be used. In a transformer-based switching regulator, the primary side of the transformer may be coupled to the unregulated DC voltage. The regulated DC output voltage is provided at the secondary side of the transformer, which is electrically isolated from the primary side of the transformer. The transformer may thus provide protective isolation of the DC output from the unregulated DC voltage.
Another issue with switching regulator configurations is that they involve a pulsed current draw from the AC power source in a manner that results in less than optimum power factor. The power factor of a system is defined as the ratio of the real power flowing to the load to the apparent power, and is a number between 0 and 1 (or expressed as a percentage, e.g. 0.5 pf=50% pf). Real power is the actual power drawn by the load. Apparent power is the product of the current and voltage applied to the load.
For systems with purely resistive loads, the voltage and current waveforms are in phase, changing polarity at the same instant in each cycle. Such systems have a power factor of 1.0, which is referred to as “unity power factor.” Where reactive loads are present, such as with loads including capacitors, inductors, or transformers, energy storage in the load results in a time difference between the current and voltage waveforms. This stored energy returns to the source and is not available to do work at the load. Systems with reactive loads often have less than unity power factor. A circuit with a low power factor will use higher currents to transfer a given quantity of real power than a circuit with a high power factor.
To provide improved power factor, some lamp ballast circuit configurations are provided with a power factor correction circuit. The power factor correction circuit may be used, for example, as a controller for controlling operation of the transistor switch in a DC-DC converter configuration such as a “flyback” converter. In such a configuration, a power factor controller may monitor the rectified AC voltage, the current drawn by the load, and the output voltage to the load, and provide an output control signal to the transistor to switch current to the load having a waveform that substantially matches and is in phase with the rectified AC voltage.